The present invention relates to a control system for an automatic transmission, wherein forward pressure reduction control is combined with torque converter slip control.
Automatic transmissions are known which comprise a torque converter and a forward friction element (for example, a forward clutch) which is kept activated by forward pressure during operation with every forward speed ratio. In these known transmissions, a so-called select shock is unavoidable upon activating the forward friction element in effecting a shift from neutral to a drive range where the first speed ratio is selectable. When a vehicle is at a half, the torque converter generates a drag torque, causing creep where the vehicle tends to move forward at a slow speed.
Since it is a fluid transmission device using hydraulic fluid as a medium, the torque converter cannot avoid slip taking place between its input and output elements, and thus has poor power transmission efficiency. In order to deal with this problem, a lock-up type torque converter has been widely used. The lock-up type torque converter is controlled such that when the torque multiplication function and torque variation absorption function are not required or unnecessary, the slip of the torque is limited or eliminated by providing slip engagement or complete engagement between the input and output elements.
As described in the publication entitled "SERVICE MANUAL RN4F02A TYPE, RL4F02A TYPE AUTOMATIC TRANSAXLE" (A261C06) issued by NISSAN MOTOR COMPANY, LIMITED, it has been the conventional practice to gradually increase the forward pressure (roller clutch activating hydraulic pressure) in order to alleviate select shock, and it has been proposed to decrease the forward pressure in order to prevent the occurrence of creep.
A conventional slip control of a torque converter is found in a KM175 type transmission manufactured by MITSUBISHI MOTOR COMPANY, LIMITED. According to this slip control, the lock-up clutch pressure is regulated so as to vary the engagement force of the lock-up clutch, thereby to limit slip in the torque converter.
These known measures, however, would pose a problem if it is intended to translate the above mentioned two controls into an electronic control because two solenoids are necessary, each for one of them. The fact that two solenoids are necessary is not advantageous from the standpoints of space saving and cost reduction.
An object of the present invention is to improve a control for an automatic transmission such that the above mentioned two controls are combined in such a manner as to save installation space and avoid cost increase.